Plate heat exchangers in which every plate formed heat transfer element consists of two plates abutting against each other are previously known. A conventional openable plate heat exchanger of this kind is shown for instance in U.S. Pat. No. 4,976,313. Owing to the fact that each heat transfer element comprises two plates it is achieved a safety against getting the two heat exchange fluids, flowing on respective sides of the heat transfer element, mixed with each other within the plate heat exchanger, if a hole would be formed through one of the plates. A leakage of one of the fluids through a hole of this kind makes the fluid in question flowing out into the space between the plates and further therethrough to and past the edges of the plates, where the leakage can be observed. So that the heat exchange fluids during normal operation of the plate heat exchanger shall not flow out into the spaces between the plates in the respective heat transfer elements the plates in each heat transfer element have to seal against each other around their said through-openings. Sealing of this kind can be obtained for instance through welding, brazing or gluing.
Even plate heat exchangers having permanently joined heat transfer elements, each comprising two plates abutting against each other, are previously known. Plate heat exchangers of this known kind, in which the double-walled heat transfer elements are joined through brazing, are shown and described in EP 517 785 and EP 527 875.
A drawback with these known brazed plate heat exchangers is that the double-walled heat transfer elements are brazed together with each other in a conventional manner, i.e. in the same manner as single-walled heat transfer elements in a brazed plate heat exchanger. The adjacent heat transfer elements are, thus, joined with each other by means of a single continuous brazing joint, and if this brazing joint is not close or does not keep tight, there is a risk that the heat exchange fluids despite the double-wall arrangement are mixed with each other in the plate heat exchanger without this being noticed.